In the recent years new set of standard for wireless communication such as WCDMA and OFDM has been introduced. This set of standard provide better spectral efficiency. However, this standards side effect is a high peek to average signals (PAR) sometime more then 10 dB. In this new standard the likelihood for a peek to occur is very low, (Usually 1xE-4 to 1xE-5).
Although those peek signals are rare, when they passing through a power amplifier they create enormous spurious emissions, both in band, and out of band. Those inter-modulations are caused by the thermal memory effect of a power amplifier. That is why the need to digital clipping has been born. Digital clipping is a mechanism that clip a rare high amplitude signals. However a direct implementation clipping will result in a spectral re-growth.
FIG. 1 illustrates a prior art system for hard clipping a signal. As could be seen, such a prior art system determines whether the input signal exceeds a given threshold (e.g. by a comparator that is configured to compare two signals), and if so—generates a correction signal which is subtracted from the input signal. If the input signal is lower than the threshold, the correction signal is zero. However, such system is not capable of soft clipping the signal, and results in many distortions to the signal.
A prior art system is known, that reduce peak to average power ratios during transmission. The Peak to average power ratios are reduced by selecting a subset of a plurality of frequencies that make up a multi-carrier symbol. Peak reduction signals, carried at the subset of frequencies, are computed to reduce the PAR of the symbol.
Another prior art technique is known for reducing peak to average power ratio (PAR) in single and multi-carrier transmitters while accounting for the effect of the transmit filters without significantly affecting a requisite transmission Power Spectral Density (PSD) mask.
Another prior art system is known to reduce the Peak power to Average power Ratio, with Forward Error Correction (FEC) mechanism. A peak detection mechanism, a procedure for choosing a subcarrier to be modified and a symbol modifier scheme are disclosed for lowering the peak power of a signal while minimizing coding errors.
Another technique is known in the art for post filtering signal peak reduction adapted for use in a multi-carrier communication system incorporating a source of a multi-carrier communication signal band limited in plural bands corresponding to the plural carriers. A first signal path receives as an input the band limited multi-carrier communication signal. A second parallel signal path includes a peak reduction calculation circuit for calculating a peak reduction correction signal and a plurality of filters providing a plurality of parallel filtering operations on the peak reduction correction signal corresponding to the plural bands to which the communication system is limited.
In another prior art system, an amplitude limiting circuit for limiting the amplitude of a signal input to a power amplifier is used, which includes an amplitude converter, determination unit, peak detector, window filter, delay circuit, and multiplier. The amplitude converter calculates the amplitude value of an input signal.
Another known proposed technique was to clip the base band signals and then filter it to prevent the spectral re-growth. However this method suffer from two drawbacks; It take tremendous amount of silicon resources (gate count) to produce a sharp slope filter (or massive DSP time), and once a sharp slope filter is applied to the Base band signal, it create ripple and group delay effect that degrade the EVM performance of the modem at low amplitude.
Therefore, effective and affordable means for applying peak to average reduction are required.